Integral covers or shrouds for steam turbine blades (usually referred to as buckets) must maintain contact from bucket to bucket in a row, to dampen vibratory stimuli and control natural frequencies. During operation, centrifugal force will cause radial growth and twisting of bucket vanes, tending to open circumferential gaps between the covers. Therefore, the covers must be assembled with enough compressive contact force between adjacent buckets to provide residual force during operation, despite the counteracting effects of centrifugal force.
Some turbine manufacturers use tangential entry buckets with internal wheel dovetails and covers, each having a rombic outline. In order to enter the wheel groove, the buckets must twist, which compresses the circumferential length of the covers. The sides of the wheel dovetail prevent untwisting of the buckets. Hydraulic rams are used to move the buckets around the wheel rim and fit them tightly together.
Other turbine manufacturers use pins to hold the buckets tightly together at assembly. Here again, it may be required to use hydraulic rams to place the covers in compression before installing such pins.
Examples of turbine bucket cover or shroud configurations may also be found in U.S. Pat. Nos. 4,840,539; 4,710,102; 3,981,615; 3,752,599; 3,572,968; 3,328,867; 3,185,441; 3,107,897; 2,942,843; 2,315,611; 2,220,918; 1,423,466; 1,247,400 and 1,149,366.
It is the object of this invention to provide an improved integral bucket cover configuration for tangential entry external wheel dovetails which maintain the necessary contact from bucket to bucket within the row.